The present invention relates to new, light-colored, urethane-modified polyisocyanates or polyisocyanate mixtures of the diphenylmethane series, a process for their production and the use of these modified polyisocyanates in the production of light-colored, tough and rigid polyurethane foams.
In the production of polyurethane foams, especially polyurethane rigid foams, polyisocyanates or polyisocyanate mixtures of the diphenylmethane series (MDI) have for many years been used as a polyisocyanate component. In the production of rigid polyurethane foams, polyisocyanate mixtures of the diphenylmethane series that have a mean NCO functionality of from about 2 to about 3.1 are preferably used. These have the disadvantage, however, of being unsuitable for the production of light-colored polyurethane foams.
The term "polyisocyanate of the diphenylmethane series" as used herein is the generic term for all di- and polyisocyanates formed by the phosgenation of aniline/formaldehyde condensates and present in the phosgenation product.
The term "polyisocyanate mixture of the diphenylmethane series" as used herein includes any mixture of "polyisocyanates of the diphenylmethane series". The phosgenation products of aniline/formaldehyde condensates, mixtures obtained by combining individual "polyisocyanates of the diphenylmethane series" and/or different mixtures thereof and also mixtures of "polyisocyanates of the diphenylmethane series" generated as distillate or distillation residue during partial distillation of phosgenation products of aniline/formaldehyde condensates.
Distilled MDI types, that is fractions with a high content of binuclear products (4,4'-isomeric MDI) or those with as low as possible a content of polynuclear isocyanates would, in principle, also be useful for the production of light-colored polyurethane foams. It is the polynuclear isocyanates of the MDI series as well as isocyanates with still higher molecular weights which produce foamed plastics having a yellowish discoloration. Color problems in polymers based on polyisocyanates of the MDI series are known from the literature.
Light-colored foamed plastics based on polyisocyanates of the MDI series have, until now, been produced only with difficulty. GB 2,207,671, U.S. Pat. No. 4,465,639 and U.S. Pat. No. 4,876,380 describe such difficult processes for producing light-colored plastics.
Isocyanates of the MDI series having a high binuclear content unfortunately have too high a reactivity and too low a functionality (absence of the polynuclear components) to produce polyurethane rigid foams of large volume and sufficient dimensional stability. The use of isocyanates containing binuclear compounds with fairly high 2,4'- and 2,2'-isomer contents, which have a suitable reactivity, is also impractical due to their low functionality. Rigid foams produced from isocyanates having such low functionality exhibit shrinkage problems which preclude their use.
Light-colored polyurethane resins are required wherever semi-opaque skins are applied together with a foamed plastic. For example, light colored resins are desirable for use in containers or vehicle bodywork with polyester skins, surfboards and refrigerators with internal plastic containers.
It is known from the literature that for applications in the field of semi-opaque skins, prepolymers based on tolylene diisocyanate (so-called TDI-prepolymers) permit the production of white, rigid foams. Nevertheless, the use of prepolymers based on tolylene diisocyanate (TDI prepolymers) is made increasingly difficult due to industrial hygiene considerations.